Reaction between a soluble ferric salt and a soluble molybdate gives a precipitate of the approximate formula Fe.sub.2 O.sub.3, 3 to 4 MoO.sub.3, .times. H.sub.2 O (X&lt;10) which is the naturally occurring form known as ferrimolybdite having an atomic ratio of Mo to Fe between 1.5 and 1.95.
The composition of precipitates artifically obtained may vary depending on the temperature at which the precipitate reaction takes place and the molecular ratio of molybdenum to iron in the starting reactants. At room temperature, the atomic ratio Mo/Fe is between 1.5 and 1.7, these narrow limits being attributable to the crystalline structure of compounds formed.
When solutions of known concentration of a soluble ferric salt and a soluble molybdate are reacted with one another, in atomic Mo/Fe ratios lower than 1.5 or higher than 2, the composition of the precipitate tends to depart from the composition of the two reacting solutions and approach that of a hydrated ferric molybdate, i.e., a ferrimolybdite. Moreover, it is known that a precipitation introduces heterogenity in going from the solution to the precipitate, the composition of which varies during the precipitation.
It has been found in accordance with this invention, that solutions of a soluble ferric salt and soluble molybdate could be reacted so that precipitation occurs partially and reversibly, that is, a pasty composition is obtained which is more or less rapidly converted to an opaque substance and then to a hard, brittle transparent substance.